1. Field of the Invention
This invention relates generally to the field of semiconductor devices. More particularly, the invention relates to methods and apparatus for finding internal charge flow patterns in dual-channel, high-electron-mobility transistors (DCHEMT) and similar devices.
2. Description of the Prior Art
A limiting factor in the switching speed of most existing field-effect transistors is the transit time, which is usually fixed by the gate length. The available lithography normally limits attempts to reduce transit times by decreasing the gate length. However, with the recent arrival of the DCHEMT, significant reductions in transit times beyond those attained by simply decreasing the gate length have been possible. Reductions in transit times are possible in the DCHEMT because electron transport takes place in the vertical as well as the usual lateral direction.
Descriptions of the conduction properties of DCHEMT's appear in: Smith et al., "A Selectively-Contacted Dual Channel High Electron Mobility Transistor," Army Science Conference Proceedings, Vol. 3, 1990, pp 347-350; and Khanna et al., "A Selectively Contacted Dual-Channel HEMT," IEEE Electron Device Letters, Vol. 10, No. 12, December 1989, pp 531-533. These publications, which are incorporated herein by reference, discuss the transport mechanisms in planar DCHEMT's and the importance of being able to predict the internal current flow patterns for accurate DCHEMT design.
Detailed descriptions of the structure and operation of a DCHEMT also appear in U.S. Pat. No. 4,821,093 issued to Iafrate et al. on Apr. 11, 1989. The Iafrate et al. patent and the Smith et al. and Khanna et al. publications teach various device patterns that achieve selective contact between the device terminals and the semiconductor channel layers, thereby producing the so called selectively contacted DCHEMT (referred to herein as the SCDC HEMT). A technique for creating selective contacts to double quantum wells appears in: Eisenstein et al., "Independently contacted two-dimensional electron systems in double quantum wells," Applied Physics Letters, 57(22), November 1990, pp 2324-2326.
As the above references show, especially the Smith et al. and Khanna et al. publications, a DCHEMT normally has two source terminals, two drain terminals, two channels, and at least one control gate. Operation of a SCDC HEMT essentially requires current to flow from both source terminals to both drain terminals. A control voltage applied to the control gate usually establishes the relative amount of current flowing at the drain terminals. This control voltage essentially controls the internal current flow distribution of the SCDC HEMT by varying the conductance of the two channels. Although this conduction mechanism has been generally recognized, it has never been known how the current input to one source terminal redistributes itself among the two drain terminals, or which of the channels carries particular source currents. Semiconductor device designers consider this information to be critical in modeling such devices and tailoring their construction to provide particular functions.